Phase shifting device

ABSTRACT

This invention relates to a phase shifting device, which generally comprises of rigid chassis, in the form of an aluminum plate, which can form a ground plane for feed lines ( 12 ) printed on a printed circuit board ( 16 ), which is mounted on the chassis, a rigid RF transparent block, which is fixed to an actuator arm and mounting bolts for slidably mounting block and actuator on the chassis. A flexi circuit, or the equivalent, ( 18 ) is attached to the block and is printed with generally c-shaped conductive strips ( 23 ), which form connections at open parts of the feed lines, in such a manner that movement of the block will cause changes in the effective lengths of the feed lines ( 12 ) and hence the relative phases of antennas fed by those feed lines.

This application is a U.S. national filing under 35 U.S.C. 371 andclaims priority from PCT/GB2003/002743, filed 27 Jun. 2003, and fromBritish Application No. 0215087.8, filed 29 Jun. 2002 (each incorporatedby reference herein).

This invention relates to a phase shifting device for an array ofantenna elements and in particular, but not exclusively, to aground-tilting antenna including such an array.

For a variety of reasons it may be desirable to induce and adjust thephase difference between signals emitted from a plurality of antennaelements in an array and one particular example of this is when thearray forms a ground tilting antenna. It is well known by designers ofwireless cell networks, such as mobile phone networks, that there is acontinuous compromise to be made between coverage, capacity and quality.Maximum coverage is achieved by emitting a horizontal beam, but inperiods of peak capacity it is found that there is often interference orcalls simply dropping off, with such an arrangement. In general,therefore, antenna are tilted downwardly by about 5°. It has, however,been appreciated that even a fixed tilt is not ideal, because it doesnot allow for changes in usage within the cell either on a short-termbasis or a long-term basis. Many aerials are therefore mounted on thesystem which can mechanically alter the tilt of the aerial, but theserequire an engineer to visit the site and they often require the antennato be switched off during adjustment.

Proposals have, accordingly, been made to alter the tilt of theradiating beam electrically by inducing phase changes along the lengthof the array corresponding to tilts of various angles. However, thesehave tended to introduce their own mechanical and control complexities.For example, in WO 01/03233 a phase shift system is described in whichthe phase is altered by altering the line length for any given antennaby varying the insertion or withdrawal of generally C-shaped conductorportions lying within, but not touching, folded conductors that formpart of the line. This requires fabrication and assembly to a finedegree of tolerance and the mechanical arrangements for achievingcontinuous adjustment of the phase in different senses in differentparts of the array in a coordinated manner are complex. Other approachesare to use moveable dielectric bodies such as described inUS-A-2002/0003458 or a slidable T-junction arrangement as described inU.S. Pat. No. 5,801,600. In each case the construction is complex andco-ordinated alteration of the phase shifts is difficult to obtain.

From one aspect the invention consists in the phase shifting device ofan array of antenna elements having respective antenna feed lines formedon a printed circuit board with respective open circuits formed therein,the device including a body slidable relative to the printed circuitboard and carrying a plurality of conductive strips for forming a RFconnection across respective open circuits, the strips being formed suchthat any given feed line is lengthened by movement of the element in onedirection and shortened by movement in an opposite direction.

Conveniently the conductive strips are generally C-shaped and there maybe one set of conductive strips which are oppositely sensed from anotherset, such that on movement in one direction, the one set of strips movesto lengthen their respective feed lines, whilst the other set shortentheir respective feed lines. The conductive strips are preferablycapacitively connected to their respective feed lines.

The body is preferably a rigid RF transparent block and the conductivestrips may be printed on the surface of the block or they may be formedon a circuit that is fixed to the block, with the body of the circuitinterposed between the block and the printed circuit board so that thereis no friction on the conductive strips to damage them. A laminationprocess may be used. Alternatively a thin dielectric sheet or coatingmay be interposed.

SUMMARY OF INVENTION

The invention further includes a phase changing assembly including aprinted circuit board for an array of antenna elements, the board havingrespective antenna element feed lines formed thereon, each feed linehaving an open circuit formed therein, a phase shifting device asclaimed in any one of the preceding claims with the body slidablymounted with respect to the printed circuit board and an actuator forcausing slidable movement.

Preferably the printed circuit board is elongate and the body ismoveable in the longitudinal axial path, which incorporates the one andthe other direction movement defined above.

The invention still further includes a ground tilting antenna arraycomprising assembly as claimed above where the antenna elements aremounted in the vertical elongate array with the upper antenna elementsconnected to the feed lines whose length is lengthened when the body ismoved in the one direction and the lower antenna elements connected tothe feed lines whose length is shortened when the body is moved in theone direction whereby a phase shift can be caused along the length ofthe array.

Although the invention has been defined above it is to be understoodthat it includes any inventive combination of the features set out aboveor in the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be performed in various ways and specific embodimentswill now be described by way of example with reference to theaccompanying drawings, in which:

FIG. 1 is a view from above of the main operative portion of a phasechanging assembly;

FIG. 2 is an enlarged view from above and to one side of the assembly;

FIG. 3 is a view from above of the printed circuit board of theassembly;

FIG. 4 is a view from above of the slider circuit of the assembly; and

FIG. 5 is a schematic view of an antenna array.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Turning to FIG. 5, the antenna elements, which form an array 1 to 10,from one side of a dual polar array are schematic illustrated. Acorresponding set (not shown) will be provided in a dual polar array toradiate the other polarization. The elements 1 to 10 are arranged inpairs and each pair (eg 1,2) radiates with the same phase. The antennaelements 1 to 10 are connected to an input 11 by bifurcating feed lines12 that include phase shifting devices 13, which are located in the feedlines 12 so that a respective individual phase shift, with respect tothe pair 5,6, can be induced in each other pair of antenna elements.Thus, in the arrangement indicated in FIG. 5, if antenna elements 5,6are taken to have zero phase, antenna elements 3,4 are shifted in thenegative sense by one phase unit, whilst 1,2 are shifted negatively bytwo phase units. Conversely, 7,8 are positively shifted by one phaseunit and 9,10 are positively shifted by two phase units. By inducing aphase distribution of this type along the array one can vary theeffective down tilt of a ground-tilting antenna, which is nominallyelectrically preset (by means of phased cable lengths) at a 5° downtilt, from between 0 to +10°.

Turning now to FIGS. 1 to 4, the Applicants' preferred construction forobtaining such a phase shift will be described.

As can best be seen in FIGS. 1 and 2, a phase shifting device isgenerally indicated at 14 and generally comprises a rigid chassis 15, inthe form of an aluminum plate, which can form a ground plane for thefeed lines 12 pinned on a printed circuit board 16 which is mounted onthe chassis 15, a rigid RF transparent block, eg of polycarbonate, or acircuit substrate 17, which is fixed to an actuator arm 19 and mountingbolts 20 for slidably mounting the block 17 and actuator 19 on thechassis 15. A circuit 18, which can be a flexi circuit can just be seenattached to the block 17; this may also be etched back into the block17.

Turning to FIG. 3, feed lines 12 are shown printed on the printedcircuit board in a conventional manner. They can be microstrip asillustrated or stripline or coplanar wave guides or any other suitabletransmission line. The lines may be printed, etched or formed on theboard 16. It will be seen that the feed lines patterns 12 are completelysymmetrical, to accommodate the dual polar antenna array and thearrangement will be described in connection with one side of the arrayonly. As with FIG. 5, a duplicate set of antenna elements, operated onthe opposite polarization, would be connected to the correspondingpoints on the other set of feed lines 12 a. As can be seen, each antennaelement is attached to a connection point, which are identified byrespective letters a to e and these corresponding points are marked onthe FIG. 5. Upstream of points a, b, d and e are respective opencircuits 21 which are constituted by a gap between two parallel sectionsof track 22. It will be noted that the parallel tracks 22, which areconnected to points a and b, point in the opposite direction to thoseconnected to points a and e and that in all cases they lie parallel tothe longitudinal axis at the elongate printed circuit board 16.

Turning to FIG. 4, the slider circuit 18 carries generally C-shapedconductive strips 23. It will be noted that each strip 23 points towardsthe centre of the slider circuit 18 and so those at the left hand sideface oppositely to those at the right hand side. As is indicated in FIG.2, this circuit 18 is adhered along the underface of the rigid block 17.Thus it is held in a position where the strips 23 overlie the arms 22 toform a capacitive RF connection across the open circuits 21 and it willfurther be understood that the degree to which the arms of the strips 23overlie the arms 22 determines the length of the feed line at thatparticular point and hence the phase shift created by the feed line.Accordingly, by sliding the rigid block 17 and hence the circuit 18axially with respect to the printed circuit board the length of the feedlines connecting to points a, b, d and e can be lengthened or shorteneddependent on the extent and direction of that movement. Thus if theblock 17 moves from right to left the feed line connecting to points aand b are extended as the strips 23 move relative to the arms 22 in themanner of a slide being pulled out on a trombone, whilst at the sametime, because of the opposite sensing of the strips 23 on the right handside of the circuit 18, the feed lines connected to points d and e areeffectively shortened, as is the air path in the trombone when the slideis pushed into the tubing. Thus when that movement takes place the phaseshifts indicated in FIG. 5 occur. As the movement is taking place with asingle block the phase change relative to each portion of the feed line12 is the same and so the stepped phase changes indicated in FIG. 5 arereadily and routinely obtained. Alternatively the phase changes arecontinuous.

It will be noted that the only movement that needs to be achieved is themovement of the single block 17 and so no complex ganging or gearingneeds to take place, nor is there any chance of mechanical wear orslackness introducing error. As the circuit 18 is adhered to the rigidblock 17 with the conductive strips 23 adjacent the block 17 all thesliding takes place on the undersurface of the circuit, which can becoated with PTFE and the strips 23 are not subjected to wear. The uppersurface of the printed circuit board 16 can also be lubriciouslyprotected by being covered with a thin PTFE layer.

The rigid block 17 can be mounted in any suitable manner, but theApplicants have found that the arrangement illustrated is particularlyconvenient. This comprises a number of bolts 20 which are screwedthrough openings in the printed circuit board 16, into the chassis 15along the central axis thereof. These bolts extend through slots 24 inthe arm 19 to define precise linear travel for the arm 19 and hence theblock 17. The bolts can be adjusted so that there is sufficient frictionbetween the underside of the slider circuit 18 and the printed circuitboard 17 for any particular position to be retained frictionally.Continuous adjustment of the phase is therefore available. Preferablythe linear movement of the arm 19 is achieved by a stepper motor (notshown) acting on a remote end thereof so that the phase shift can beadjusted remotely, so the effective ground tilt angle of the array canbe achieved, either at ground or, even more preferably, from a remotecontrol station. Often the adjustment will be made to reflect changingtraffic profiles over a period of weeks or months, but the system isequally capable of allowing changing angles throughout a pre-set dailypattern, in the manner of traffic light delays, so that, for example,antennas near roads, carrying rush hour traffic, may require a greaterdown tilt during peak periods than at other times or, it could be a realtime adjustment which reflects the traffic being handled by anyparticular array at any particular time.

The circuit illustrated is designed for 1710 to 2170 MHz widebandoperation, when connected to wideband antenna elements. However, it canbe scaled to other frequency bands eg 800 MHz to 1 GHz, by those skilledin the art.

1. A phase shifting device for an array of antenna elements havingrespective antenna feed lines, formed on a printed circuit board, withrespective open circuits formed therein, the device comprising: a bodyslidable relative to the printed circuit board and carrying a firstplurality of conductive strips and a second plurality of oppositelysensed conductive strips forming an RF connection across respective opencircuits, the first and second plurality of conductive strips beingformed at spaced locations on the body, wherein when the body moves in afirst direction a phase length associated with a first open circuit ofthe first plurality of conductive strips is lengthened and a phaselength associated with a second open circuit is shortened, the secondopen circuit being overlaid by the second plurality of conductivestrips.
 2. The device as claimed in claim 1 wherein at least one of thefirst and second plurality of conductive strips are capacitivelyconnected to their respective feed lines.
 3. The device as claimed inclaim 1 wherein the body is a rigid RF transparent block.
 4. The deviceas claimed in claim 3 wherein at least one of the first and secondplurality of conductive strips are printed, etched or formed on asurface of the block.
 5. The device as claimed in claim 3 wherein atleast one of the first and second plurality of conductive strips aremounted on a circuit that is fixed to the block with the body of thecircuit interposable between the block and the printed circuit board. 6.The device as claimed in claim 1 further including a low friction thindielectric layer interposed between engaging surfaces of the board andthe conductive strips.
 7. The device as claimed in claim 1 furthercomprising an actuator for causing the slidable movement and wherein theantenna element is slidably mounted with respect to the printed circuit.8. The device as claimed in claim 7 wherein the printed circuit board iselongate and the body is movable in a longitudinally axial path.
 9. Aground tilting antenna array comprising: a phase shifting devicecomprising: a body slidable relative to a printed circuit board; and, aplurality of oppositely sensed antenna elements mounted to the circuitboard in a vertical elongate array and defining upper antennae and lowerantennae, the upper antennae being connected to feed lines that arelengthened when the body is moved in a first direction and the lowerantenna elements being connected to the feed lines that are shortenedwhen the body is moved in the first direction, whereby a phase shift iscaused along the length of the array.